The present invention relates generally to electrical machines, and more particularly, to synchronous machines with permanent magnets.
Synchronous machines with permanent magnets find application in various electric drives fed from inverters due to their high efficiency and no need for rotor cooling. These two advantages, along with minimum rotor maintenance need (no slip rings and brushes), make a permanent magnet (PM) synchronous machine the choice number one in numerous applications, when compared with a wound rotor synchronous machine.
The better utilization of rotor volume in a PM machine has, however, a price. The induced voltage in the stator winding of a wound rotor synchronous machine can be controlled by the field current This possibility does not exist in a conventional PM synchronous machine, since the magnetization of permanent magnets is constant. When the speed of a wound rotor synchronous machine increases, its stator induced voltage can be kept constant or decreased by decreasing the field current. When, however, the speed of a PM synchronous machine increases, its stator induced voltage increases proportionally. At certain speed the stator induced voltage reaches the maximum allowed amount, determined by winding insulation properties and inverter voltage capability. Above that speed the drive cannot operate safely due to risk of equipment damage.
Another characteristic of a conventional PM synchronous machine is its constant number of poles. This is not always the case in a squirrel cage induction machine, the rotor of which has as many poles as its stator. By changing the number of stator poles in a squirrel cage induction machine, the number of rotor poles changes too, which enables machine generate torque at any pole number. This way, the speed range of a squirrel cage induction machine can be extended:
In an arbitrary ratio by using separate winding for each polarity;
In a ratio 2:1 with Dahlander connected stator winding;
In a ratio p: (pxc2x12) with pole-amplitude modulated (PAM) stator winding, where p stands for the number of poles
In an arbitrary ratio with pole-phase modulated (PPM) stator winding, as shown in U.S. Pat. No. 5,977,679.
A conventional PM rotor has a rigid magnetic structure which does not allow any change of the number of poles. Therefore, the speed range of a conventional synchronous machine with PM rotor cannot be increased by changing the number of its poles. Any attempt to change the number of stator poles without changing the number of rotor poles of an electric machine results in machine malfunctioning. A PM machine with different number of stator and rotor poles draws excessive currents from the source, without delivering any useful mechanical torque on the shaft.
When loaded, a conventional PM synchronous machine demonstrates another shortcoming: the load currents in the stator winding create their own magnetic field which distorts the field of permanent magnets (armature reaction). Since the field of permanent magnets has a constant amplitude and is fixed to the rotor surface, the load current distorts the resulting air gap field. This way, the stator induced voltage becomes a function of the load current, limiting possible applications of a conventional PM synchronous machine as a generator.
The present invention is directed to overcoming the problems related to rigid rotor magnetization and single-speed capability in conventional PM machines. Briefly summarized, both the shape and magnitude of magnetic field distribution along the rotor circumference are controlled in this invention by means of current(s). The control current(s) can flow only during time interval in which a new magnetic state is created, or permanently. When the rotor magnetization is controlled by additional stator currents the stator of the proposed machine draws during regular run only the load current.
A rotor of a synchronous machine has iron segment (1) and a plurality of tangentially magnetized permanent magnets (2), (3) per pole with different radial dimensions. On the stator side a conventional AC winding carries stator currents during normal operation. During short remagnetization phase an additional component of stator current provides for change of magnetization direction in a portion of longer magnets.
A rotor of a synchronous machine has two iron segments (4), a wedge (24), and a trapezoidally shaped permanent magnet (5) per pole. A conventional AC winding on the stator side carries during a short period of time an additional current component, which remagnetizes a portion of the magnet closer to the rotor bore. The radial height of the remagnetized magnet portion is proportional to the remagnetizing current.
A rotor of a synchronous machine has optional squirrel cage bars (25), along with several trapezoidally formed iron yoke segments (6) and rectangular permanent magnets (7) per pole. The stator (8) is slotted, whereas the slots (9) carry a pole changing Dablander winding, or a pole-amplitude modulated (PAM) winding, or a pole-phase modulated (PPM) winding, as described in U.S. Pat. No. 5,977,679.
When the stator number of poles is changed, the stator winding draws temporarily an additional component of current which remagnetizes the rotor magnets in such a manner that the number of rotor poles is equal to the number of stator poles. The optional rotor cage generates torque which can bring the rotor into new synchronous speed after changing the number of poles.
A rotor of a synchronous machine has optional squirrel cage bars (26), along with several rectangular iron yoke segments (10) and trapezoidal permanent magnets (11) per pole. The stator (12) is slotted, whereas slots (13) carry a pole changing Dahlander winding, or a pole-amplitude modulated (PAM) winding, or a pole-phase modulated (PPM) winding, as described in U.S. Pat. No. 5,977,679.
When the stator number of poles is changed, the stator winding draws temporary an additional component of current which remagnetizes the rotor magnets in such a manner that the number of rotor poles is equal to the number of stator poles. The optional rotor cage generates torque, which can bring the rotor into new synchronous speed after changing the number of poles. After changing the number of poles, the stator winding can carry during a short period of time an additional current component which remagnetizes portions of the magnets closer to the rotor bore The size of remagnetized magnet portion is proportional to the amplitude of remagnetizing current.
A rotor of a synchronous machine has two magnets per pole, the main magnet (14) and the auxiliary magnet (15), as well as iron segments (16). The main magnet provides for machine excitation. The auxiliary magnet compensates for the armature reaction field created by the stator current.
A rotor of a synchronous machine has one magnet per pole (18), one iron segment per pole (19), and several independently driven coils per pole (17). The coil currents are chosen so to create the magnetic field which acts in the same direction as the magnet field, or to he spatially shifted to the magnet field. In the former case, the coil current helps control the amount of induced voltage in the stator winding. In the latter case, the coil current can compensate for armature reaction of the stator winding.
A rotor of a synchronous machine has one tangentially (21) and one radially (23) oriented magnet per pole, several independently fed coils (20), and iron segments (22). The three sources of magnetic flux (two magnets and coils) combine their action in such a manner to fully compensate for effects of stator armature reaction.